(i) Field of the Invention
This invention relates to a bubble measurement cell, and to a method for measuring size distribution of gas bubbles in a liquid.
(ii) Description of the Prior Art
In column flotation, there are numerous ways of generating bubbles. However, knowledge of the size distribution of gas bubbles in a liquid produced is critical in assessing the relative merits of bubble generators and in investigations into the mechanics of the flotation procedure. The need, therefore, is to measure size distribution of gas bubbles in a liquid in a lamella of known dimensions located within the flotation column, close to, but away from, the walls of the flotation column.
Techniques for determining the presence or absence of gas bubbles in liquids are known. Apparatus for determining the onset of the formation of bubbles, i.e., cavitation or boiling, but not the proportion of bubbles, have been described by a number of patentees.
In U.S. Pat. No. 3,381,525 to Kartluke et al., sound waves were launched into a liquid. The liquid was monitored for sound waves at subharmonic frequencies of the launched sound waves. When subharmonic waves were detected, cavitation is imminent or has begun.
In U.S. Pat. No. 3,240,674 to Ledwidge, a similar technique was used. No sound waves were added to the liquid. Instead, the frequencies of sound waves in the liquid were monitored for a selected spectrum peak that indicated localized boiling, a prelude to boiling of the entire liquid volume.
U.S. Pat. No. 3,622,958 to Tucker et al, disclosed a number of methods of detecting the existence of gas bubbles in a liquid. Waves at a fundamental frequency were launched into a liquid by a first transducer and waves at harmonic frequencies are detected by a second transducer. Detection of harmonic frequency signals indicated the presence of gas bubbles. Alternatively, reflected waves at harmonic frequencies were detected by the same transducer that launches the fundamental frequency wave. In still another embodiment, multiple frequency sound waves were launched into the liquid which was monitored for waves having frequencies equal to a sum or difference of two of the frequencies of the launched waves.
Many other patents purport to provide a solution to the problem of measurement of the size distribution of gas bubbles in a liquid.
U.S. Pat. No. 3,529,234 patented Sep. 15, 1970 to R. D. Keen, provided a method and apparatus for solving the problem of detecting bubbles or vapor within the liquid metal system or liquid metal droplets within a vapor system. The patentee used a high frequency oscillator which was placed adjacent the pipe containing the flowing liquid or vapor. An oscillator was tuned for resonance coupling at the characteristic resonance absorption frequency of the liquid. Energy absorption by the liquid was indicated by either the oscillator current measurement or by a sensor coil or antenna which is connected to a detector or radio receiver. The receiver or oscillator current measurement, when calibrated, indicated discrete vapor or bubble count in a liquid metal system or the percentage of liquid-vapor flow, vapor quality in a vapor system. Bubbles were also detected, and the size of bubbles can be determined. The tank coil of a high frequency oscillator was placed about a pipe containing the flowing fluid which may be in either a liquid or vapor phase or combination thereof. At a particular frequency, resonance coupling occurred between the coil and the nuclei of the fluid, resulting in an absorption of energy by the nuclei. The amount of energy absorption depended upon the cross sectional area of the fluid as modified by any vapor bubbles, so that the output from a detecting circuit indicated the bubble content of a liquid fluid system or the vapor quality or amount of liquid in a vapor system.
U.S. Pat. No. 3,738,154 patented Jun. 12, 1973 to R. E. Henry, provided a method of measuring entrained gas in a liquid. In the patented method, a choked converging-diverging nozzle was employed in a method of detecting the presence, and measuring the volumetric concentration, of entrained gas in a liquid. The liquid-gas mixture was accelerated through the nozzle to critical flow conditions and the pressure at the throat of the nozzle was measured. The temperature and pressure of the mixture of the stagnation region were monitored, the throat pressure of the liquid-gas mixture being a function of only the void fraction at any given stagnation temperature and pressure.
U.S. Pat. No. 4,418,565 patented Dec. 6, 1983 by P. A. St. John, provided an ultrasonic bubble detector. In the patented device, the ultrasonic bubble detection apparatus utilized a typically one-piece, rigid housing having a channel defined therein for receiving flow tubing in which bubbles were to be detected. First and second ultrasonic sending and receiving transducers were positioned on opposite sides of the channel, with an aperture communicating between each of the transducers and the channel, the aperture being filled with an elastomeric material capable of transmitting ultrasound energy between the channel and each transducer means. An air-containing slot was positioned at the bottom of the channel to hinder the propagation of ultrasound energy through the housing from the first to the second transducer by a route other than one passing through the elastomeric material.
U.S. Pat. No. 4,763,525 patented Aug. 16, 1988 to W. N. Cobb, provided apparatus and method for determining the quantity of gas bubbles in a liquid. In the patented method, an ultrasonic wave was launched into the mixture and magnitudes of two reflected waves were measured, and compared. The logarithm of the magnitudes of the reflected waves was a measure of the quantity of gas content.
U.S. Pat. No. 4,862,729 patented Sep. 5, 1989 by K. Toda et al, provided a method for measuring the amount of the gas contained in liquid. The patented method included introducing a liquid material into a vacuum measuring chamber, changing the volume of the measuring chamber to provide two different liquid pressures of the liquid material in the measuring chamber, and detecting the different pressures to measure the amount of gas on the basis of Boyle's law.
Thus, as pointed out above, the known technology does not provide a simple, reliable method of quantitatively measuring the bubble content of a liquid-gas mixture. In order to avoid the problem of direct measurements, the prior art has relied instead on secondary, indirect measurements, assumptions, and calculations (e.g., using known rates of air injection, known water volume, etc.), and assuming no (or some fractional rate of) coalescence of bubbles.
In a photographic system for directly measuring the size distribution of gas bubbles in a liquid, the main limitation is the photographic process required for recording and analysis; the film known by the Trade-mark KODACHROME film now in use requires five working days for processing. Also, even though the image analysis process now in use works quite well, it needs some refinement to provide optimum results.